An optocoupler comprises an optical signal transmitter, for example a light-emitting diode or laser diode, and an optical signal receiver, for example a photo-diode or photo-transistor. The signal transmitter is connected to a first electrical circuit and the signal receiver to a second electrical circuit. A signal from the first circuit is converted to an optical signal in the signal transmitter. This optical signal is transmitted in the form of optical radiation to the signal receiver, where it is converted to an electrical signal and then supplied to the second circuit. Galvanic separation between the two electrical circuits is obtained with the aid of an optocoupler.
The optocoupler is generally designed as one component including the signal transmitter and the signal receiver, suitable radiation-transfer means, for example in the form of light guides or mirrors and electrically insulating means to galvanically separate the two circuits.
Both information and power can be transmitted from one circuit to the other with the aid of an optocoupler.
Separate optocouplers may be used for the different signals if the signals are to be transmitted in both directions. Alternatively a single optocoupler may be used for a two-way transmission of signals and/or power, in which case each of the optical components is designed to operate optionally as an emitter or receiver of optical radiation.
An optocoupler is often required to have small dimensions. It is then suitably produced in the form of a monolithic optocoupler, using the integrated-circuit technique. The two electro-optical components are then arranged on a common substrate. The layers which together form the two components are successively produced during manufacturing on the substrate by means of liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), for instance.
The substrate may consist of a good insulator, for example, sapphire. It has also been found to be advantageous to make a substrate--of a "semi-insulating" material, preferable of the same type as the material of which the two components are made, for example gallium arsenide. The resistivity of a semi-insulating material may be within the range 10.sup.7 -10.sup.12 ohmcm.
Optocouplers of this type are previously known from U.S. Pat. No. 4,212,020 and PCT application publication number WO 85/04491.
An optocoupler is already known through U.S. Pat. No. 4,021,834, in which both optical components are applied on a common substrate. This consists of gallium arsenide and has a strongly n-doped, and thus low-resistive, part extending between the two components. The two components are electrically insulated from each other since they are arranged on SiO.sub.2 layers on the surface of the substrate. Since, in practice, these layers can only be made thin, thus their break-through voltage will be low, and this known optocoupler can therefore only be used for extremely low voltages. Therefore, in order to meet realistic demands for electrical insulation between the two components, a different category of optocoupler must be used, namely an optocoupler with an insulating or semi-insulating substrate. The interference problems which the present invention aims at solving occur in such optocouplers. However, these problems are not mentioned in the U.S. patent specification.
The above-mentioned publication also describes how a modulating signal source can be connected to the substrate with the aid of metal contacts applied on both sides of the substrate with the object of using the optocoupler as a phase modulator.
In monolithic optocouplers, the optical radiation can be transmitted in the known manner from the transmitting to the receiving component in various ways. Mirroring or total-reflecting members may be arranged to ensure that as much radiation from the emitter as possible reaches the receiver. Alternatively, a transparent layer acting as a waveguide may be arranged between the two components. According to another alternative, the substrate may be used as a waveguide for the optical radiation, and utilized as such.
It has been found in monolithic optocouplers that interference may be transmitted between the two electrical circuits connected to the optocoupler.